This invention relates in general to solar tracking and, more particularly, to dual mode solar tracking.
To implement the proper operation of photovoltaic, solar-thermal, and various instrumentation systems, accurate and continuous tracking of the sun is necessary. Numerous systems have been devised and some have achieved considerable success. Typically, these systems employ paired photocells which are mounted on flat surfaces at equal and opposite angles of inclination to a base. The cells are also equiangularly disposed about a reference axis which extends perpendicularly from the base. The reference axis constitutes the aiming direction of the tracking system.
The underlying theory of operation is that the illumination of each mounting surface is proportional to the cosine of the angle of incidence of radiation. Thus, when the system is trained on the sun, incident solar radiation is parallel to the reference axis, the cells are equally illuminated and produce equal electrical outputs if they are properly matched.
The outputs of the cells may be fed to a bridge circuit for comparison, and when the cells are equally illuminated, the bridge output is zero. When the sensor is not aimed at the sun and incident radiation is at an angle to the reference axis, one cell receives more illumination than the others, and the bridge produces an output signal which can be utilized to reorient the sensor.
It is also conventional to provide an opaque shield which is generally positioned at or above the apex of the inclined photocell mounts. The shadow cast by the opaque shield on the lesser illuminated cells further reduces their output thereby effecting a significant gain in precision, especially when incident radiation is at a relatively small angle to the reference axis. This, of course, is the situation when the sensor is only slightly misdirected from a position aiming at the sun.
Some problems do arise with such tracking systems, however. Perhaps their greatest fault is their susceptibility to environmental conditions which cause them to react to spurious light sources. These sources may include clouds from which the sun may be reflected and ground albedo. It is possible, of course, to exclude most spurious sources by narrowing the field of view of the sensor but in addition to the equipment complications that such elements introduce, they are ineffective against spurious sources which are not necessarily fixed in position.
Another factor which tends to detract from the performance of systems of the prior art is the unequal aging of the photocells, whose outputs must remain matched if accurate error signals are to be produced. Cell aging and, for that matter, moving spurious sources, can be compensated for by means of frequently repeated calibration operations, but the equipment for such calibration is complex and expensive. Still another problem arises with compensated tracking systems, and that is excessive power consumption. This last problem is, of course, exacerbated when that power must be generated locally, the size and mass of the system necessarily being increased.
Accordingly, the major object of the present invention is to improve solar tracking systems by utilizing calculated tracking rates modified by sensors which differentiate between direct and diffuse light.
A further object of the present invention is to simplify solar tracking by avoiding the requirements of maintaining matched photocells in the sensing system.
A still further object of the present invention is to reduce power consumption in solar tracking systems by operating the tracking system at a relatively constant rate.